In distillation, absorption and other chemical-separating processes and techniques, various types of vapor-liquid contacting trays, such as sieve trays, jet trays, valve trays and the like, are employed, in order to provide for intimate contact between an upwardly flowing vapor and a downwardly flowing liquid in a vapor-liquid contacting column. A wide variety of tray designs and concepts has been proposed, generally directed around improving the tray efficiency, improving the tray capacity, or improving the tray mechanical structure. However, experience has shown that these design concepts often work against each other; for example, a tray design for high efficiency typically sacrifices tray capacity and also complicates the tray mechanical structure. Furthermore, claimed tray designs of improved tray efficiency or capacity are not easily and convincingly verified in pilot-plant tests or in commercial tray towers.
It is desirable to minimize the overhead of a tray design, wherein a typical operating tray generally comprises a highly active heat mass-transfer region; that is, the froth region across the horizontal surface of the tray, a poor heat mass-transfer region; that is, the droplet disengagement region above the froth region and underneath the next tray, and the nonactive downcomer regions, where little or no vapor-liquid contact occurs. Trays designed for high liquid-loading chemical applications generally have to bear a heavy downcomer overhead.
A multiple downcomer tray has been developed which employs a sieve-type tray with multiple downcomers, with the downcomer inlet width between, for example, 1 to 4 inches, and with the downcomer outlet terminating at an elevation above the downcomer inlet of the next tray below, with a liquid seal of the downcomer provided by using orifices, spouts of screen materials (see, for example, U.S. Pat. Nos. 3,410,540, 4,159,291, 4,278,621, and 4,297,329). By terminating the downcomer above the froth on the tray below, the tray area occupied by the downcomer may be reduced by 50%. This additional tray deck can be used as an active bubbling area; for example, by putting additional valves on the tray deck below the dowmcomer, and thereby increasing the capacity of the tray. In addition downcomer back-up flooding, a major tray-capacity limiting factor in high liquid-loading applications, also can be avoided or reduced. The multiple downcomer tray, thus, provides for a high percentage of available tray area and a lower tray spacing in the column, and, for high liquid-loading applications, is equivalent to a higher tray capacity and higher efficiency per unit tower height; for example, tray spacing of 10 to 12 inches, as compared to 20 to 24 inches for a typical valve tray.
However, such sieve-type multiple-downcomer trays, for example, having a sieve-type tray deck and slotted downcomers, have an inherently poor turndown ratio. The normal turndown ratio for the valve distillation trays can be greater than 6, while for sieve trays it is usually around 3, while with multiple-downcomer sieve trays, the turndown ratio is often 2 or less. Trays have been developed employing a flexible downcomer design (see U.S. Pat. No. 3,784,175). This patent attempts to improve the poor turndown ratio of the multiple-downcomer sieve tray, while maintaining a positive downcomer seal, by employing a movable member. However, because of the questionable reliability of the movable member, such trays have not been widely accepted in the marketplace.
Some trays are composed of a plurality of plates made from a highly porous, open-pored, sintered material, particularly for use in the absorption and/or desorption of gases by liquids. Such plates are usually composed of a sintered magnesium or aluminum silicate. The plates often have a plurality of generally parallel passage openings of predetermined cross-sections extending therethrough, and include a plurality of run-off means comprised of tubes which have, at the lower end thereof, a gas shut-off device, to block the upward flow of gas in the downcomer (see, for example, U.S. Pat. No. 3,958,964). These sintered materials have very small openings, are subject to plugging, and are restricted to low liquid flow rates; therefore, the sintered materials are much different from the packing materials. Furthermore, the objective of using the sintered materials in the downcomer is to prevent gas flow up the downcomer, rather than to promote good heat and mass transfer between gas and liquid in the downcomer.
Therefore, there exists a need for an improved vapor-liquid contact tray of simple mechanical structure, but which tray can achieve high tray efficiency and high tray capacity and a wide turndown ratio, through a reduction in tray overhead.